This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9903219-5 filed in Sweden on Sep. 8, 1999; the entire content of which is hereby incorporated by reference.
The present invention relates to oscillators, specially a voltage-controlled oscillator, preferably an oscillator realized in MMIC (Monolithic Microwave Integrated Circuit) technique. The oscillator comprises a first substrate, on which a resonator circuit and an amplifier circuit are arranged. The resonator circuit comprises first set of components and said amplifier circuit comprising a second set of components and an amplifier transistor.
Some contradictory compromises must be made when designing broadband oscillators with low phase noise. Also, the limiting characteristics of the components constituting the oscillator must be taken into consideration.
The phase noise of an oscillator is given by the so-called Leeson""s equation (1):                                           L            ⁡                          (                              f                m                            )                                =                                                    1                2                            ⁡                              [                                  1                  +                                                            1                                              f                        m                        2                                                              ⁢                                                                  (                                                                              f                            0                                                                                2                            ⁢                            Q                                                                          )                                            2                                                                      ]                                      ⁢                                          F                ⁢                                  xe2x80x83                                ⁢                k                ⁢                                  xe2x80x83                                ⁢                T                            P                        ⁢                                          (                                  1                  +                                                            f                      c                                                              f                      m                                                                      )                            ⁢                              xe2x80x83                            [                              dBc                /                Hz                            ]                                      ⁢                  
                ⁢                                            Where                                                                                            f                    m                                    =                                      offset                    ⁢                                          xe2x80x83                                        ⁢                    frequency                    ⁢                                          xe2x80x83                                        ⁢                    from                    ⁢                                          xe2x80x83                                        ⁢                    the                    ⁢                                          xe2x80x83                                        ⁢                    oscillation                    ⁢                                          xe2x80x83                                        ⁢                    frequency                                                  ,                                                                                        xe2x80x83                                                                                                          f                    0                                    =                                      oscillation                    ⁢                                          xe2x80x83                                        ⁢                    frequency                                                  ,                                                                                        xe2x80x83                                                                                      F                  =                                      phase                    ⁢                                          xe2x80x83                                        ⁢                    noise                    ⁢                                          xe2x80x83                                        ⁢                    of                    ⁢                                          xe2x80x83                                        ⁢                    the                    ⁢                                          xe2x80x83                                        ⁢                    reflection                    ⁢                                          xe2x80x83                                        ⁢                    amplifier                                                  ,                                                                                        xe2x80x83                                                                                      k                  =                                      Boltzmann                    ⁢                                          xe2x80x83                                        ⁢                    constant                                                  ,                                                                                        xe2x80x83                                                                                      T                  =                  temperature                                ,                                                                                        xe2x80x83                                                                                                          f                    c                                    =                                      switching                    ⁢                                          xe2x80x83                                        ⁢                    frequency                    ⁢                                          xe2x80x83                                        ⁢                    for                    ⁢                                          xe2x80x83                                        ⁢                                          1                      /                      f                                        ⁢                                          xe2x80x83                                        ⁢                    noise                                                  ,                                                                                        xe2x80x83                                                                                      Q                  =                                      Q                    ⁢                                          -                                        ⁢                    factor                    ⁢                                          xe2x80x83                                        ⁢                    for                    ⁢                                          xe2x80x83                                        ⁢                    resonator                    ⁢                                          xe2x80x83                                        ⁢                    circuit                                                  ,                                  xe2x80x83                                ⁢                and                                                                                        xe2x80x83                                                                    P                =                                  input                  ⁢                                      xe2x80x83                                    ⁢                  power                  ⁢                                      xe2x80x83                                    ⁢                  of                  ⁢                                      xe2x80x83                                    ⁢                  the                  ⁢                                      xe2x80x83                                    ⁢                  reflection                  ⁢                                      xe2x80x83                                    ⁢                                      amplifier                    .                                                                                                          (        1        )            
Every oscillator is a periodically time-varying system, and the time varying nature of it must be considered when modelling phase noise. The noise source in the circuit can be divided into two groups: device noise such as thermal, shot and flicker noise and interface such as substrate and supply noise.
When designing a resonator circuit for a voltage-controlled oscillator (VCO), usually varactor diodes are used as voltage controlled capacitors. Advantageously, these are realised on GaAs (Gallium Arsenide) or Si (Silicon) substrates. However, GaAs is preferred as a considerably better Q-value is obtained for the resonator circuit as a whole. This is due to the fact that both varactor diodes and planar inductor coils have superior performance on GaAs relative to Si. Especially the resonator circuit usually comprises an inductor, which is a metallic coil directly arranged on the semiconductor surface. Since it is advantageous to arrange the inductor coil on an insulating substrate, a GaAs substrate which is substantially insulating is more suitable than a Si substrate which is semi-insulating. The same applies to the case of a microstrip transmission line resonator. A resonator with variable resonance is described in the Swedish patent application No. 9900850-0, xe2x80x9cVaractor Coupled High-Q Monolithic Resonatorxe2x80x9d (Resonator Application).
The differences between GaAs and Si are specially outstanding for frequencies above some GHz, which in fact prevents employment of Si for producing resonant LC structures thereon.
As mentioned above, the oscillator comprises a second amplifier part which preferably is a reflection amplifier. In a preferred embodiment which is based on a transistor, the reflection amplifier has an amount of amplification that is needed to overcome the losses of the resonator and thus obtain a self oscillation. An output is arranged on an appropriate point of the amplifier and connected to the signal chain.
Known transistor techniques on GaAs are, e.g. MESFET (Metal-Schottky Field Effect Transistor), PHEMT (Pseudomorphic High Electron Mobility Transistor) and HBT (Heterojunction Bipolar Transistor). Generally, PHEMT offers good amplification at high frequencies, MESFET has low cost of production and HBT has high efficiency, positive voltage supply and good linearity.
Generally, on Si CMOS and bipolar processes are used. The development of the Si transistors has resulted in applicable transistors for frequencies up to 10 GHz. The SiGe technique provides much higher cut off frequencies and it performance can seriously be compared to~the GaAs processes. Also, with regard to the price, the Si-based processes have considerable advantages.
A demand for a transistor to be used in an oscillator is that the transistor has a low I/f-noise. Consequently, this low frequency noise is converted through the non-linearity of the circuit to phase noise. Consequently, also the non-linear characteristics of the transistor which are of interest are effected. Mainly, the I/f noise is a surface phenomenon meaning that transistors having vertical structure, such as bipolar transistors, have a lower I/f noise than surface oriented transistors, such as MESFET and PHEMT. Typically, the switch frequency for I/f noise for the different transistor techniques range from  greater than 1 MHZ for GaAs PHEMT and MESFET,  greater than 100 kHz for GaAs HBT and  less than 10 kHz for Si BJT. The result is that on GaAs usually HBT technique is preferred when producing oscillators. However, the yet lower switching frequency of the bipolar Si transistors could additionally reduce the phase noise if it was possible to use these types of transistors.
Among the GaAs HBTs, there are two main groups with different material in the emitter layer: AlGaAs/GaAs-HBT and InGaP/GaAs-HBT. Different manufacturers use different material. In HBTs with AlGaAs appear so-called deep electron traps, due to the aluminum content. The traps are actuated by heat and trap and release electrons with some certain time constants, which results in a disturbance in the uniform flow of the electrons. The disturbance appears as noise, which assumes Lorentz formed spectra in the frequency range of 10 kHz to 1 MHZ. This is an additional low frequency noise contribution which can be converted to the phase noise in an oscillator and it is normally called generation-recombination-noise (g-r-noise) or xe2x80x9cburst noisexe2x80x9d. The temperature dependency of the nose results in temperature variations in the phase burst in an oscillator made of said type of transistors. HBTs with InGaP in the emitter layer are normally free from mentioned type of electron traps. However, a manufacturer uses one of the processes, which means that it is not always possible to choose a specific transistor type.
The deep electron traps are absent in the silicon-based transistors thus making them free from the g-r-noise, which is an advantage when designing oscillators.
The known solutions to the problem of making broadband VCOs, specially in MMIC technique are:
As the realisation of hyper abrupt varactor diodes on an MMIC is not possible through any of the known standard processes, usually they are located outside the chip. Normally, the entire resonator circuit 110 (FIG. 1) is arranged outside the chip 100 to obtain a better Q factor. The production is more expansive owing to the additional costs for the varactor diode. The performance,does not improve compared to a construction with the entire resonator on a single chip. Neither it is possible to use the encapsulated varactors at the frequencies above about 5 GHz due to theparasite reactances of the capsule, as the varactor diodes in chip form are difficult to bond to.
It is also possible to arrange the entire VCO on a Si or SiGe, if so-called 3D technique is used, in which dielectric layers are arranged on top of the chip, and a new ground plane and above it a new conducting layer are provided. Thus, it is possible to realise inductors with low losses. Nevertheless, the varactor diodes are still in the silicon and accordingly they are inferior to diodes in GaAs. Consequently, the entire solution tends to become inferior.
EP 523 564 describes an improved oscillating circuit for use in microwave frequency bands having reduced power loss and smaller in vertical size. The local oscillating circuit includes an MMIC oscillator comprising a FET and a resonator connected thereto so as to stabilize the oscillating frequency of the oscillator. The resonator is ring-shaped and arranged as close as several xcexcm to several tens of xcexcm to a predetermined position of a micro strip line forming a feedback loop connected to the FET forming the oscillator. Moreover, the resonator is a thin film formed by depositing a high-temperature superconducting material. As exemplar embodiments, YBCO, niobium and the like, can be used as high-temperature superconducting materials. Furthermore, a portion of the micro strip line, closest to the resonator, is concentrically disposed therewith to form a circular arc portion whose central angle is set at 90 degrees.
The object EP 893 878 is to provide a high-frequency oscillating circuit that does not have its characteristics such as a S/N ratio degraded by an external electromagnetic interference. The bases of a first and a second oscillating transistor are connected together directly or via a capacitor having a sufficiently low impedance at an oscillating frequency and wherein a differential signal output is obtained from between the emitters of the first and second oscillating transistors as an oscillating output. A resonator, varactor diodes, and capacitors and chokes constituting a resonating circuit for an oscillating circuit IC are integrated together as a module separate from a negative-resistance generating circuit including oscillating transistors configured as an IC.
EP 627 812 relates to a voltage controlled planar oscillator having a microwave transistor as the active component and a frequency-determining switching network connected thereto, which contains a varactor diode and a dielectric resonator. To be able to integrate such an oscillator monolithically on as small a chip as possible, it is proposed that the dielectric resonator is coupled to the switching network via a first microstrip line and that the microwave transistor is connected to one end of the first microstrip line with a first one of its three gates via the varactor diode.
According to DE 195 07 786 an oscillator has a superconductive resonator and at least one device-connected conductor, between first and second multilayer substrates and having a planar structure. A resonator is fixed on the second substrate; and the substrates are mutually aligned and joined and comprise, respectively, a first GaAs layer on which a conductor or the resonator is attached, an intermediate protective layer, pref. of Si3N4, and a second YBa2Cu3O7xe2x80x94delta layer.
For facilitating a compact and inexpensive preparation without lowering the Q of a resonator by forming a tuning circuit inside an MMIC and composing the resonator of an external resonance circuit connected to that, JP 829 37 28 discloses a MMIC voltage controlled oscillator (MMIC-VCO) includes the tuning circuit for frequency modulation into an MMIC and forms only a microstrip resonant line or a dielectric resonant element consisting of the resonator on the external circuit of the MMIC. This microstrip resonant line or dielectric resonant element and the MMIC are connected by wire bonding or ribbon bonding. Therefore, since the microstrip resonant line or dielectric resonant element to be the resonator is provided as the external resonant circuit, the Q of the resonant circuit is not lowered and when preparing the MMIC-VCO, it is not necessary to newly prepare another tuning circuit.
The main objective of the present invention is to provide an arrangement which combines the advantages of two different techniques, e.g. GaAs and Si, and thereby provides a solution to above-mentioned problems and drawbacks.
The arrangement according to the present invention provides a broadband oscillator circuit which has very low or even no phase noise at all, low 1/f noise and a very high Q-factor compared to the known devices.
For these reasons, in the initially mentioned oscillator, the first substrate comprises at least two additional substrates: a second substantially insulating substrate and a third substantially semiconducting substrate, and in that at least said transistor is arranged on said third substrate while said first and second set of components are arranged on said second substantially insulating substrate.
In the most preferred embodiment, the transistor and said second set of components are arranged on said at least third substrate. The advantage of this embodiment is that small variations in, the bonding wire results in small variations in the phase displacement, which yields small variations in the phase but insignificant variations in the phase noise.
Preferably, the transistor and/or said second set of components on said third substrate are connected to said resonator on said second substrate by means of bonding wire.
Preferably, second substrate is one of Gallium Arsenide (GaAs), Indium Phosphide (InP), Gallium Nitride (GaN), Indium Arsenide (InAs), metamorphous techniques as a thin layers of InP on a wafer of GaAs or different types of filed effect transistor techniques and said third substrate (230) is one of Silicon (Si), Silicon Germanium (SiGe), Silicon Carbide (SiC) or the like.
Preferably, but not exclusively said amplifier is a reflection amplifier. The second set of components comprises a first and second feedback capacitors wherein said first capacitor connects the emitter of the transistor to its base and said second capacitor connects an output signal terminal to ground. In one embodiment, the resonator comprises an inductor in parallel with a resonator capacitor, comprising two anti-serially connected first and second varactor diodes, the varactor diodes being connected through their anodes to a scanning voltage through which the resonator capacitor is varied. The resonator further comprises a capacitor connected to the a third varactor diode for coupling the resonator to the amplifier.
Preferably, a first carrier or supporting member includes one of said second or third substrates.
The invention also concerns a method of arranging an oscillator comprising a first substrate on which a resonator circuit and an amplifier circuit. The resonator circuit comprises first set of components and said amplifier circuit comprising a second set of components and an amplifier transistor. The method comprises the steps of: arranging on said first substrate at least two additional substrates: a second substantially insulating substrate and a third substantially semiconducting substrate; arranging at least said transistor on said at least third substrate while said first and second set of components arrange on said second substantially insulating substrate. The method further comprises the step of arranging said transistor and said second set of components on said third substrate.